DALLAS (AP) — The Latest on the death in Texas of a U.S. Customs and Border Protection agent (all times local):

8:15 p.m.

A U.S. official with knowledge of the investigation into death of a border patrol agent says the surviving agent who radioed in for help doesn't remember what happened.

The official, who was briefed on the investigation but is not authorized to speak publicly and spoke on the condition of anonymity, said Monday the late Rogelio Martinez was found at the bottom of a 14-foot culvert and that investigators believe he may have fallen. The official said the incident happened after dark in an area is known for drug activity and where agents often look for drugs in culverts.

A spokeswoman for the FBI, which is leading the investigation, said Monday in a statement that both agents were found in a culvert near Van Horn, Texas, and that both had traumatic head injuries.

Spokeswoman Jeanette Harper said results of Martinez's autopsy are pending.

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AP writer Elliot Spagat in San Diego contributed to this report.

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6:15 p.m.

A U.S. border patrol official says unconfirmed reports that an agent who died and another who was injured were attacked are "speculation" as of Monday evening.

U.S. Customs and Border Patrol official Rush Carter says all the agency can confirm is that the two agents were injured and that a joint investigation with the FBI will hopefully determine how that happened.

Carter, a supervisor for the CBP region where 36-year-old Rogelio Martinez died, emphasized that the investigation is ongoing.

Chris Cabrera, a spokesman for a border patrol agents union, said Monday that agents who responded to the scene Sunday described it as "grisly" and said the dead agent and his partner had "extensive injuries" from being struck with a rock or rocks.

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4:10 p.m.

A union official says a U.S. border agent who died while on patrol in West Texas appears to have been struck in the head with a rock or rocks.

Chris Cabrera, a spokesman for the National Border Patrol Council, said Monday that U.S. Customs and Border Protection agents who responded to the scene Sunday described it as "grisly" and said the dead agent, 36-year-old Rogelio Martinez, and his partner had "extensive injuries."

A Border Patrol spokesman said Martinez died at a hospital and that his partner, whose name hasn't been released, was seriously injured.

It has been more than 24 hours, yet there hasn't been an official explanation of what happened. Several elected officials have said the agents were attacked, including President Donald Trump, who renewed his call for a border wall between the U.S. and Mexico.

The incident or attack happened near Van Horn, which is about 30 miles (48 kilometers) from the Mexico border and about 110 miles (177 kilometers) southeast of El Paso.

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12:45 p.m.

Texas is offering a $20,000 reward for information that leads to the arrest and conviction of the person or people who killed a U.S. border agent and injured another.

Gov. Greg Abbott announced the reward in a statement Monday in which he expressed gratitude to U.S. Customs and Border Protection agents.

Agent Rogelio Martinez was killed and his partner was seriously injured Sunday in what elected officials have said was an attack near Van Horn, which is about 30 miles (48 kilometers) from the Mexico border and about 110 miles (177 kilometers) southeast of El Paso. Martinez's partner, whose name hasn't been released, is hospitalized in serious condition.

Investigators haven't released certain details about the case, including what time it happened and whether they're pursuing specific suspects. An FBI spokeswoman did say the agents were not shot.

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11:45 a.m.

President Donald Trump has offered condolences to the family of a Border Patrol agent who was killed in an apparent attack in West Texas.

Trump says a second officer injured Sunday was "brutally beaten and badly, badly hurt" but "looks like he'll make it."

The president spoke at a Cabinet meeting Monday at the White House. He says the wall he has promised to build along the border between the U.S. and Mexico is on the agenda.

Trump says the wall is necessary because "it's rough territory" down there and a lot of things are happening along the border, including drug trafficking. He pledged that "we're going to straighten it out."

The Border Patrol has released few details about what led to the death of 36-year-old Agent Rogelio Martinez, including whether investigators have any suspects or think smugglers or people who were in the country illegally were involved.

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8:50 a.m.

Texas Gov. Greg Abbott and Sen. Ted Cruz have characterized it as an attack that caused the weekend death of a border agent and injuring of a second.

The Republican senator said in a statement Sunday that 36-year-old Agent Rogelio Martinez died as a result of the attack earlier that day near Van Horn, which is about 30 miles from the Mexico border and 110 miles southeast of El Paso. He said the nation is grateful "for the courage and sacrifice of our border agents."

Abbott also described the incident as an attack in a tweet Sunday.

The Border Patrol hasn't released many details about what happened. It said in a statement that the agents "were responding to activity" while on patrol near Interstate 10.

The FBI has taken over the investigation.

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12:30 a.m.

The FBI is investigating after one U.S. Customs and Border Protection agent died and another was seriously injured while on duty in South Texas.

Border Patrol spokesman Douglas Mosier says Agent Rogelio Martinez and his partner were taken to a hospital Sunday after they were hurt while responding to activity in the Big Bend area. The 36-year-old Martinez died at the hospital.

The agency has not released details about how the two agents were injured.

Border Patrol records show Big Bend accounted for about 1 percent of the more than 61,000 apprehensions its agents made along the Southwest border between October 2016 and May 2017. The region's mountains and the Rio Grande make it a difficult area for people to cross illegally into the U.S. from Mexico.